A page is a unit of virtual memory that holds 4 KB of
data and can be transferred between real and auxiliary storage.

A paging space, also called a swap space, is a logical
volume with the attribute type equal to paging. This type of logical volume is
referred to as a paging space logical volume or simply paging space. When the
amount of free real memory in the system is low, programs or data that have not
been used recently are moved from real memory to paging space to release real
memory for other activities.

The installation creates a default paging logical
volume (hd6) on drive hdisk0, also referred as primary
paging space. The default paging space size is determined during the system
customizing phase of AIX installation according to the following standards:

Paging space can use no less than 16 MB except for hd6, which can use no
less than 32 MB in AIX Version 4.2.1 and later.

Paging space can use no more than 20 percent of the total disk space.

If real memory is less than 32 MB, paging space is two times real
memory.

If real memory is greater than or equal to 32 MB, paging space is real
memory plus 16 MB.

The amount of paging space required by an application
depends on the type of activities performed on the system. If paging space runs
low, processes may be lost. If paging space runs out, the system may panic.
When a paging space low condition is detected, additional paging space should
be defined. The system monitors the number of free paging space blocks and
detects when a paging space shortage exists. The vmstat command
obtains statistics related to this condition. When the number of free paging
space blocks falls below a threshold known as the paging space warning level,
the system informs all processes (except the kernel process) of the low paging
space condition.

The I/O from and to the paging spaces is random and is
mostly one page at a time. The vmstat command reports indicate the
amount of paging space I/O is taking place. A sample output of the
vmstat command is shown in Figure 64.

Figure 69: vmstat Command Output

To improve paging performance, you should use multiple paging spaces and
locate them on separate physical volumes whenever possible. However, more than
one space can be located on the same physical volume.

The general recommendation is that the sum of the sizes
of the paging spaces should be equal to at least twice the size of the real
memory of the machine up to a memory size of 256 MB (512 MB of paging space).
For memories larger than 256 MB, the following rule is recommended:

Total paging space = 512 MB + (memory size - 256 MB) * 1.25

Ideally, there should be several paging spaces of roughly equal size each on
a different physical disk drive. If you decide to create additional paging
spaces, create them on physical volumes that are more lightly loaded than the
physical volume in rootvg.

While the system is booting, only the primary paging space (hd6)
is active. Consequently, all paging-space blocks allocated
during boot are on the primary paging space. This means that the primary paging
space should be somewhat larger than the secondary paging spaces. The secondary
paging spaces should all be of the same size to ensure that the round-robin
algorithm can work effectively.

The lsps -a command provides a snapshot of the current utilization
level of all the paging spaces on a system.

A volume group that has a paging space volume on it cannot be varied off or
exported while the paging space is active. Before deactivating a volume group
having an active paging space volume, ensure that the paging space is not
activated automatically at system initialization and then reboot the system.